The present invention relates to a seal stack for use in a plunger-type positive displacement water pump that operates in high temperature/high pressure conditions. More specifically, the present invention relates to a seal stack that has longer life and better reliability than those currently available.
Plunger-type water pumps are positive displacement pumps that operate by a plunger reciprocating in a cylinder in such a manner that water is drawn into the cylinder on an upstroke of the plunger and water is forced out of the cylinder on a down stroke of the plunger. Pressure activated one-way valves define the ports through which the water ingresses and egresses from the cylinder. When pumping liquids such as water using a plunger pump, the desired high pressures are generated due to the incompressibility of the liquid.
In order to create the pressures desired, it is necessary to have efficient seals in the pump at all locations where leakage is a potential. One such location for potential leakage is the interface between the plunger and the housing walls. As mentioned above, high pressures are desired, thus the importance of the seal is magnified. To insure effective operation, high pressure seals must be used around the plunger to prevent water from leaking between the plunger and the cylinder walls. Typically, a seal stack is used such as that shown in FIG. 1, to provide this seal.
FIG. 1 shows a prior art seal stack made up of several components. At one end the first component of the seal stack is a head ring 1. The head ring 1 is a hard molded plastic such as polyacetal and has a male chevron shape. The function of this head ring will be explained after all of the various components are introduced. The next component is pressure packing 2. The pressure packing 2 is made of NBR with a rubber fabric base. It is shaped with a female chevron on one side and a male chevron on the opposite side. The female chevron is configured to mate with the male chevron of the head ring 1.
Next in the stack is a restop ring 3. Restop ring 3 is also shaped with a female chevron on one side and a male chevron on the opposite side, the female chevron being configured to mate with the male chevron of the pressure packing 2.
Next is the first brass component, intermediate ring 4. Intermediate ring 4 includes a female chevron, which is configured to mate with the male chevron of the restop ring 3. An upper side (as shown in FIG. 1) of the intermediate ring 4 has a flat surface. Leakage holes are formed in the side walls of the intermediate ring. These leakage holes are placed to feed any water that does seep past the pressure packing 2 back into the inlet.
The next component is a low pressure seal 5. The low pressure seal is constructed of HNBR which is a molded soft rubber. The low pressure seal 5 has a lower end having a plurality of raised flat surfaces that form channels therebetween. The flat surfaces abut the flat surface of the intermediate ring 4. The low pressure seal 5 has a ridge with a slight overhang on its upper side. This overhang provides a snap fit with the next component, a low pressure support packing 6. The low pressure support packing 6 has a groove configured to mate with the overhanging ridge to form a snap fit therebetween such that the low pressure seal 5 and the low pressure support packing 6 snap together to form one component.
The final component is a low pressure brass ring 7. The low pressure brass ring 7 includes a female chevron that completely receives the low pressure seal and the low pressure support packing such that, when assembled, the flat surfaces of low pressure seal 5 are nearly flush with the outer edge of the brass ring 7. The low pressure brass ring 7 also includes an O-ring around the outside of it to form a low pressure seal on the inside of the cylinder.
Once assembled the seal stack is placed on one end of a cylinder within a cylinder block. The plunger passes through the concentric openings on the center of each components making up the seal stack. The interior dimensions of the various openings are nearly identical with the internal diameter of the pressure packing 2 and the low pressure seal assembly being the smallest. Thus, the contact between the inside of the pressure packing 2 and the plunger form the seal. The rest of the components of the seal stack function to support this contact as outlined below.
In operation, the end of head ring 1 acts against a top ledge of the cylinder. This causes the male chevron of the head ring 1 to press into the female chevron of the pressure packing 2 providing support therefore. Support for the pressure packing 2 in the opposite direction is provided by the restop ring 3. The restop ring 3 acts as a cushion between the pressure packing 2 and the intermediate brass ring 4. Notably, not all manufacturers incorporate a restop ring 3. The intermediate ring 4 supports the seal and provides leakage holes to redirect any water that made its way past the high pressure packing 2 back into the inlet. The low pressure seal assembly 5 and 6 also act against the plunger. However, the pressures encountered by this assembly are not as great as those encountered by the pressure packing. The low pressure seal provides a backup seal against the plunger. The low pressure brass ring provides support for the low pressure seal assembly and also provides a rubber O-ring which seals the seal stack against the inside of the cylinder head (not shown).
The prior art seal stack works because the pressure packing 2 forms a tight fit against the plunger. However this can be problematic. The tight fit between the pressure packing 2 and the plunger requires constant contact with cool water to prevent the seal stack from overheating. When fluid is not present however, overheating problems exist. Thus this seal stack realistically has no run-dry capability. If the pump runs dry, the pressure packing quickly heats up causing the restop ring 3 to melt and the pressure seal 2 to deteriorate and fail.
In addition to the dry run problem mentioned above, the operating life of this seal stack is too short. During normal operations one can expect to get about 1,000 hours of useful life out of this seal stack. The pumps on which the seal stack are used are typically employed often suffer significantly if they are shut down to replace seal stacks. Thus, product life is very important. At the end of its useful life the pressure packing first begins to deteriorate, specifically, the NBR wears off of the fabric on the inside of the packing. Seal failure is then eminent.
The third problem encountered by this seal stack is that the seal stack does not have the capability to be used in high temperature operations. As explained above, the seal stack requires continuous contact with cool water. Obviously, operating the seal stack to pump high temperature water (>165° F.) results in a significant heating problems. This leads to a decreased life span, on the order of approximately 400 hours.
In light of the above issues related to seal stacks, it is the object of the present invention to provide a seal stack that has a run-dry capability. Such a seal stack will have the ability to avoid overheating problems that have plagued prior art seal stacks when run dry.
It is also an object of the present invention to provide a seal stack that has a longer life span. Longer life will reduce required maintenance by the user.
Yet another object of the present invention is to provide a seal stack that can be used in high temperature operations.